refactor hsl_to_rgb a touch

This rewrites the existing logic to expose more of the symmetries,
and especially to make them clearly identical subexpressions.

I think it's clear that the intent in hue_to_rgb is to wrap the t value
back into 0-1... that's t = fract(t).

No GM diffs.

Change-Id: I9d62d8f80bcb45711ee334f953d3f6410e068ce4
Reviewed-on: https://skia-review.googlesource.com/14940
Reviewed-by: Herb Derby <herb@google.com>
3 files changed